The effect of SARS-CoV-2 testing on healthcare seeking behaviour at primary care level: implications for COVID-19 vaccine effectiveness estimates in test-negative design studies.

BackgroundDiagnostic (self-)testing for SARS-CoV-2 may lead to selection bias in test-negative case-control designs (TND) for COVID-19 vaccine effectiveness (CVE) at primary care level. We investigated whether after the acute phase of the pandemic, (self-)testing among those with an acute respiratory infection (ARI) was associated with healthcare seeking behaviour at primary care level in the general Dutch population. MethodsWe pooled questionnaire data from three study rounds (June 2022, November 2022 & April 2023) of the nationwide PIENTER Corona cohort study. Among participants aged 18-91 years, we selected the first self-reported ARI episode, defined as cough, sore throat, dyspnoea and/or coryza, since March 2022. We performed log-binomial regression analyses adjusted for age, sex, educational level and comorbidities to assess associations between COVID-19 vaccination, SARS-CoV-2 (self-)testing and general practitioner (GP) consultation, and between GP consultation and prior (self-)test result. ResultsAmong 3152 participants with an ARI episode, vaccinated (vs unvaccinated) participants more often (self-)tested (adjusted RR [95% CI]: 1.07 [1.04-1.11]) or consulted a GP (1.57 [1.21-2.09]). (Self-)test result overall was not associated with GP consultation (0.86 [0.69-1.08]). Vaccination-stratified analyses showed vaccinated individuals were less likely to consult the GP after a positive (self-) test (0.62 [0.49-0.79]), while unvaccinated were more likely to (2.00 [1.08-3.51]). ConclusionsIn this Dutch population-based cohort, GP consultation between May 2022 and March 2023 was differential by (self-)test result and vaccination status, indicating potential selection bias in TND CVE estimates from testing before GP consultation. More research to quantify this bias in various settings is needed.